As one technique for the gas chromatographic analysis, a technique called the “comprehensive two-dimensional GC” or “GC×GC” is commonly known (for example, refer to Patent Literature 1).
In the comprehensive two-dimensional GC, various components contained in a target sample are initially separated with a column as the first dimension (which is hereinafter called the “primary column”). The thereby eluted components are introduced into a modulator. The modulator repeats the operation of catching the introduced components at regular intervals of time (which is normally within a range from a few seconds to one dozen seconds; this interval of time is hereinafter called the “modulation time”) and releasing those components with an extremely narrow time bandwidth into a column as the second dimension (which is hereinafter called the “secondary column”). In general, the component separation in the primary column is performed under such a condition that the elution occurs at a rate approximately equal to or slightly low than the rate applied in a commonly used GC. On the other hand, as compared to the primary column, the secondary column has a different polarity and a smaller inner diameter, and the component separation is performed under such a condition that each elution will be completed within the specified modulation time.
In this manner, in the comprehensive two-dimensional GC, a plurality of components which have not been separated by the primary column and whose peaks overlap each other can be separated in the secondary column, whereby the separation performance is dramatically improved as compared to normal GCs. Therefore, comprehensive two-dimensional GCs are extremely effective for an analysis of a sample which contains a number of compounds whose retention times are close to each other, such as a hydrocarbon analysis of diesel fuel.
A similar technique to the comprehensive two-dimensional GC is also known in the field of liquid chromatographic analysis, i.e. the comprehensive two-dimensional LC or LC×LC, which uses two columns having different separation characteristics. In the present description, both the comprehensive two-dimensional GC and the comprehensive two-dimensional LC are collectively called the “comprehensive two-dimensional chromatograph”.
These comprehensive two-dimensional chromatographs detect the components in a sample gas or sample solution which has passed through the two columns. Therefore, the data produced by the detector is a sequence of data arranged in time-series order. Accordingly, by plotting the obtained data in order of generation, a one-dimensional chromatogram as shown in FIG. 4A can be created, which is similar to a chromatogram obtained with a normal GC, i.e. which has the horizontal axis indicating the retention time and the vertical axis indicating the signal intensity. In FIG. 4A, tm denotes the modulation time. The section of the chromatogram within this time (tm) is the chromatogram which reflects the state of separation of the components in the secondary column.
As noted earlier, in may cases, the two columns in the comprehensive two-dimensional chromatograph have different separation characteristics. Therefore, to show the state of separation in each column in an easy-to-understand form, a two-dimensional chromatogram having two orthogonal axes which respectively indicate the retention time in the primary column and the retention time in the secondary column is created, with the signal intensity represented by contour lines, color scale, or gray scale. FIG. 4B illustrates the order in which the data are arrayed to create the two-dimensional chromatogram from one-dimensional chromatogram data. The range of the vertical axis of this graph corresponds to the modulation time tm. The one-dimensional chromatogram data are sequentially plotted upward from the lower end (0) along the vertical axis (the solid arrow in FIG. 4B). After reaching tm, the plotting point is shifted rightward along the horizontal axis and returned to the lower end of the vertical axis (the broken line in FIG. 4B), after which the upward-plotting operation along the vertical axis is repeated. Consequently, a two-dimensional chromatogram as shown in FIG. 4C is obtained. In FIG. 4C, the signal intensity is indicated by contour lines.
FIG. 5 is one example of the two-dimensional chromatogram based on actually measured data in a comprehensive two-dimensional GC. In this example, the signal intensity is represented by a color scale (although the gray-scaled representation is used in the drawing, since colored drawings are not allowed). In the case of a temperature-programmed analysis in which the temperature of the columns is increased with time, the horizontal axis in the two-dimensional chromatogram represents the order of the boiling point, while the vertical axis represents the order of polarity. Therefore, the analysis operator can easily understand the nature of each component on the basis of the two-dimensional chromatogram. Even when many components are contained in the sample, the analysis operator can intuitively understand what kinds of components are contained.
As a data processing software product for creating such a two-dimensional chromatogram, the “GC Image” offered by GC Image LLC is commonly known (see Non Patent Literature 1).
As disclosed in Non Patent Literature 2, in order to identify or quantify various components in a sample which contains a comparatively high amount of foreign substances, it is useful to combine the comprehensive two-dimensional chromatograph having the previously described high level of separation performance with a mass spectrometer, and in particular, a mass spectrometer capable of MS/MS analyses, such as a triple quadrupole mass spectrometer or ion-trap time-of-flight mass spectrometer. For example, with the triple quadrupole mass spectrometer, it is possible to select, as the precursor ion, an ion having a specific mass-to-charge ratio originating from a compound, fragment the precursor ion by a collision induced dissociation process, and perform an exhaustive detection (i.e. scan measurement) of various product ions produced by the fragmentation. This is the MS/MS analyzing technique called the “product ion scan measurement”. By this technique, fragments (ion species) which result from the breakage of the bonds at various sites on a specific chemical structure can be investigated.
FIG. 6 shows one example of the MS/MS spectrum obtained by the product ion scan measurement. In FIG. 6, the peak of the precursor ion, which actually cannot be detected, is shown in the broken line.
Additionally, the triple quadrupole mass spectrometer is capable of performing other MS/MS analysis techniques, such as a precursor ion scan measurement for exhaustively investigating precursor ions which produce a specific kind of product ion, or a neutral loss scan measurement for investigating the combination of the precursor ion and product ion which produce a specific kind of electrically neutral fragment (“neutral loss”) through the fragmentation.
Accordingly, as in the case of screening drugs or poisons, when a large number of compounds which have comparatively similar chemical structures (e.g. which have the same basic skeleton and merely differ from each other in the kind of substituent group) need to be simultaneously identified, it is preferable to use a comprehensive two-dimensional chromatograph having a triple quadrupole mass analyzer as its detector in such a manner that, after the large number of compounds are sufficiently separated in the temporal direction as well as according to their mass-to-charge ratios, the product ion scan measurement or neutral loss scan measurement is performed, and the thereby collected data are analyzed and processed to discern the difference in their partial chemical structure.
In recent years, MS/MS analyses in triple quadrupole mass spectrometers or ion-trap time-of-flight mass spectrometers have become increasingly faster in operation and more complex in procedure. For example, there is an apparatus having the function of: automatically determining the mass-to-charge ratio, signal intensity and other properties of a peak which has appeared on a measured mass spectrum; automatically selecting a precursor ion which conforms to a predetermined condition; and performing a product ion scan measurement for the selected ion, instead of a product ion scan measurement for a precursor ion having a previously specified mass-to-charge ratio. Such a function is commonly known as the IDA (intelligent data acquisition, information-dependent acquisition, etc.), auto-MS/MS, or by other names.